The present invention relates generally to lithography, and more particularly, to an apparatus and method for compensating for pressure exerted on the lithography lens caused by the immersion fluid.
A lithography apparatus is used to transfer images defined by a reticle or other patterning element such as a programmable mirror array (see U.S. Pat. Nos. 5,296,891, 5,523,193 and PCT applications 98/38597 and 98/33096 for example, all incorporated by reference herein) onto a semiconductor wafer during fabrication. A typical lithography apparatus includes an illumination source, a reticle stage assembly for positioning the reticle, a wafer stage for supporting the wafer, and an optical assembly including lenses for projecting the image defined by the reticle onto the wafer. Control and measurement systems are also provided to control the movement of the wafer and measure the position of the wafer relative to the optical assembly respectively.
Immersion lithography systems utilize a layer of immersion fluid that fills a gap between the final lens of the optical assembly and the wafer. The fluid enhances the resolution of the system by enabling exposures with numerical apertures (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with light that would otherwise be totally internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the immersion fluid are possible. Fluid immersion also increases the depth of focus, which is the tolerable error in the vertical position of the wafer, compared to a conventional lithography system. Immersion lithography thus has the capability of providing resolution down to 50 nanometers or lower.
One potential issue with immersion lithography is that fluid pressure on the lens may cause the last lens of the optical assembly to become displaced. More specifically, the amount of force on the lens depends on the pressure exerted by the fluid and the size of the surface area of the lens, lens mount hardware, and any immersion fluid supply nozzles attached to the lens mount hardware.
The fluid pressure may be caused by a number of reasons. With immersion lithography, the surface tension of the liquid at the air-fluid interface surrounding the exposure area, sometimes referred to as the meniscus, has the effect of sucking or pulling down the lens and optical assembly. Variations in the amount of immersion fluid may also cause pressure variations on the lens. The applicants have found that a positive or negative change of only 0.02 cubic centimeters will cause a change of force of approximately 50 milli-newtons on the lens with an 80 millimeter diameter. Also as water escapes the gap, the water flow also has a tendency to create a pull down force on the lens. Dynamic motion of the wafer in the horizontal plane can cause shear forces that exert pressure on the lens. Vertical motions of the wafer performed for focusing purposes may also cause unwanted vibrations or vertical coupling of the lens.
Excessive forces exerted on the last lens of the optical assembly can cause a number of problems during exposure operation. If the force causes the lens to be displaced, the resulting image projected onto the wafer may be out of focus. On the other hand, if the optical assembly is too rigidly mounted to prevent the displacement, lens aberrations may result due to thermal expansion, again resulting in a blurring of the projected image.
An apparatus and method for compensating for pressure exerted on the lithography lens caused by the immersion fluid is therefore needed.